8 – <i>N</i> Rule and Chemical Bonding in Main-Group MgAgAs-Type Compounds

The chemical bonding of main-group MgAgAs-type compounds is analyzed with quantum chemical direct-space techniques. A new bonding concept is developed that unites the former ionic bonding and polyanionic network models. Polar and nonpolar contributions to the bonding are extracted by the combined analysis of electron density and electron localizability. A direct-space representation of the 8 – <i>N</i> rule is introduced. In this approach, the anions’ heteropolar bonds are treated as a superposition of covalent (nonpolar) and lone-pair closed-shell (polar) contributions. The relation between covalent (nonpolar) and lone-pair (polar) character is obtained with the ELI-D/QTAIM basin intersection technique. This ratio depends on the constituting elements. On basis of this approach, MgAgAs-type compounds are compared with Zintl phases, where covalent bonds and lone pairs are spatially separated

Turning Gold into “Diamond”: A Family of Hexagonal Diamond-Type Au-Frameworks Interconnected by Triangular Clusters in the Sr–Al–Au System

A new homologous series of intermetallic compounds containing three-dimensional (3-d) tetrahedral frameworks of gold atoms, akin to hexagonal diamond, have been discovered in four related Sr–Au–Al systems: (<b>I</b>) hexagonal SrAl_3–<i>x</i>Au_4+<i>x</i> (0.06(1) ≤ <i>x</i> ≤ 0.46(1), <i>P</i>6̅2<i>m</i>, <i>Z</i> = 3, <i>a</i> = 8.633(1)–8.664(1) Å, <i>c</i> = 7.083(2)–7.107(1) Å); (<b>II</b>) orthorhombic SrAl_2–<i>y</i>Au_5+<i>y</i> (<i>y</i> ≤ 0.05(1); <i>Pnma</i>, <i>Z</i> = 4, <i>a</i> = 8.942(1) Å, <i>b</i> = 7.2320(4) Å, <i>c</i> = 9.918(1) Å); (<b>III</b>) Sr₂Al_2–<i>z</i>Au_7+<i>z</i> (<i>z</i> = 0.32(2); <i>C</i>2<i>/c</i>, <i>Z</i> = 4, <i>a</i> = 14.956(4) Å, <i>b</i> = 8.564(2) Å, <i>c</i> = 8.682(1) Å, β = 123.86(1)°); and (<b>IV</b>) rhombohedral Sr₂Al_3–<i>w</i>Au_6+<i>w</i> (<i>w</i> ≈ 0.18(1); <i>R</i>3̅<i>c</i>, <i>Z</i> = 6, <i>a</i> = 8.448(1) Å, <i>c</i> = 21.735(4) Å). These remarkable compounds were obtained by fusion of the pure elements and were characterized by X-ray diffraction and electronic structure calculations. Phase <b>I</b> shows a narrow phase width and adopts the Ba₃Ag_14.6Al_6.4-type structure; phase <b>IV</b> is isostructural with Ba₂Au₆Zn₃, whereas phases <b>II</b> and <b>III</b> represent new structure types. This novel series can be formulated as Sr_<i>x</i>[M₃]_1–<i>x</i>Au₂, in which [M₃] (= [Al₃] or [Al₂Au]) triangles replace some Sr atoms in the hexagonal prismatic-like cavities of the Au network. The [M₃] triangles are either isolated or interconnected into zigzag chains or nets. According to tight-binding electronic structure calculations, the greatest overlap populations belong to the Al–Au bonds, whereas Au–Au interactions have a substantial nonbonding region surrounding the calculated Fermi levels. QTAIM analysis of the electron density reveals charge transfer from Sr to the Al–Au framework in all four systems. A study of chemical bonding by means of the electron-localizability indicator indicates two- and three-center interactions within the anionic Al–Au framework

Classical and Nonclassical Germanium Environments in High-Pressure BaGe₅

A new crystalline form of BaGe₅ was obtained at a pressure of 15(2) GPa in the temperature range from 1000(100) to 1200(120) K. Single-crystal electron and powder X-ray diffraction patterns indicate a body-centered orthorhombic structure (space group <i>Imma</i>, Pearson notation <i>oI</i>24) with unit cell parameters <i>a</i> = 8.3421(8) Å, <i>b</i> = 4.8728(5) Å, and <i>c</i> = 13.7202(9) Å. The crystal structure of <i>hp</i>-BaGe₅ consists of four-bonded Ge atoms forming complex layers with Ge–Ge contacts between 2.560(6) and 2.684(3) Å; the Ba atoms are coordinated by 15 Ge neighbors in the range from 3.341(6) to 3.739(4) Å. Analysis of the chemical bonding using quantum chemical techniques in real space reveal charge transfer from the Ba cations to the anionic Ge species. Ge atoms having nearly tetrahedral environments show an electron-localizability-based oxidation number close to 0; the four-bonded Ge atoms with a Ψ-pyramidal environment adopt a value close to 1-. In agreement with the calculated electronic density of states, the compound is a metallic conductor (electrical resistivity of ca. 240 μΩ cm at 300 K), and magnetic susceptibility measurements evidence diamagnetic behavior with χ₀ = −95 × 10^–6 emu mol^–1

Making and Breaking Bonds in Superconducting SrAl_4–<i>x</i>Si_<i>x</i> (0 ≤ <i>x</i> ≤ 2)

We explored the role of valence electron concentration in bond formation and superconductivity of mixed silicon–aluminum networks by using high-pressure synthesis to obtain the BaAl₄-type structural pattern in solid solution samples SrAl_4–<i>x</i>Si_<i>x</i> where 0 ≤ <i>x</i> ≤ 2. Local ordering of aluminum and silicon in SrAl_4–<i>x</i>Si_<i>x</i> was evidenced by nuclear magnetic resonance experiments. Subsequent bonding analysis by quantum chemical techniques in real space demonstrated that the strong deviation of the lattice parameters in SrAl_4–<i>x</i>Si_<i>x</i> from Vegard’s law can be attributed to the strengthening of interatomic Al–Al and Al–Si bonds within the layers (perpendicular to [001]) for 0 ≤ <i>x</i> ≤ 1.5, followed by the breaking of the interlayer bonds (parallel to [001]) for 1.5 < <i>x</i> ≤ 2 and leading to the structural transition from the BaAl₄ structure type with three-dimensional anionic framework at lower <i>x</i> values to the two-dimensional anion of the BaZn₂P₂ structure type with increasing <i>x</i> values. Low-temperature measurements of the resistivity and heat capacity reveal that SrAl_2.5Si_1.5 and SrAl₂Si₂ prepared at high pressures exhibit superconductivity with critical temperatures of 2.1 and 2.6 K, respectively

New Monoclinic Phase at the Composition Cu₂SnSe₃ and Its Thermoelectric Properties

A new monoclinic phase (<i>m2</i>) of ternary diamond-like compound Cu₂SnSe₃ was synthesized by reaction of the elements at 850 K. The crystal structure of <i>m2</i>-Cu₂SnSe₃ was determined through electron diffraction tomography and refined by full-profile techniques using synchrotron X-ray powder diffraction data (space group <i>Cc</i>, <i>a</i> = 6.9714(2) Å, <i>b</i> = 12.0787(5) Å, <i>c</i> = 13.3935(5) Å, β = 99.865(5)°, <i>Z</i> = 8). Thermal analysis and annealing experiments suggest that <i>m2</i>-Cu₂SnSe₃ is a low-temperature phase, while the high-temperature phase has a cubic crystal structure. According to quantum chemical calculations, <i>m2</i>-Cu₂SnSe₃ is a narrow-gap semiconductor. A study of the chemical bonding, applying the electron localizability approach, reveals covalent polar Cu–Se and Sn–Se interactions in the crystal structure. Thermoelectric properties were measured on a specimen consolidated using spark plasma sintering (SPS), confirming the semiconducting character. The thermoelectric figure of merit <i>ZT</i> reaches a maximum value of 0.33 at 650 K

Dumbbells of Five-Connected Ge Atoms and Superconductivity in CaGe₃

CaGe₃ has been synthesized at high-pressure, high-temperature conditions. The atomic pattern comprises intricate germanium layers of condensed moleculelike dimers. Below <i>T</i>_c = 6.8 K, type II superconductivity with moderately strong electron–phonon coupling is observed

Phase Range of the Type-I Clathrate Sr₈Al_<i>x</i>Si_{46–<i>x</i>} and Crystal Structure of Sr₈Al₁₀Si₃₆

Samples of the type-I clathrate Sr₈Al_<i>x</i>Si_{46–<i>x</i>} have been prepared by direct reaction of the elements. The type-I clathrate structure (cubic space group <i>Pm</i>3̅<i>n</i>) which has an Al–Si framework with Sr²⁺ guest atoms forms with a narrow composition range of 9.54(6) ≤ <i>x</i> ≤ 10.30(8). Single crystals with composition A₈Al₁₀Si₃₆ (A = Sr, Ba) have been synthesized. Differential scanning calorimetry (DSC) measurements provide evidence for a peritectic reaction and melting point at ∼1268 and ∼1421 K for Sr₈Al₁₀Si₃₆ and Ba₈Al₁₀Si₃₆, respectively. Comparison of the structures reveals a strong correlation between the 24<i>k</i>-24<i>k</i> framework sites distances and the size of the guest cation. Electronic structure calculation and bonding analysis were carried out for the ordered models with the compositions A₈Al₆Si₄₀ (6<i>c</i> site occupied completely by Al) and A₈Al₁₆Si₃₀ (16<i>i</i> site occupied completely with Al). Analysis of the distribution of the electron localizability indicator (ELI) confirms that the Si–Si bonds are covalent, the Al–Si bonds are polar covalent, and the guest and the framework bonds are ionic in nature. The Sr₈Al₆Si₄₀ phase has a very small band gap that is closed upon additional Al, as observed in Sr₈Al₁₆Si₃₀. An explanation for the absence of a semiconducting “Sr₈Al₁₆Si₃₀” phase is suggested in light of these findings

Phase Range of the Type-I Clathrate Sr₈Al_<i>x</i>Si_{46–<i>x</i>} and Crystal Structure of Sr₈Al₁₀Si₃₆

Samples of the type-I clathrate Sr₈Al_<i>x</i>Si_{46–<i>x</i>} have been prepared by direct reaction of the elements. The type-I clathrate structure (cubic space group <i>Pm</i>3̅<i>n</i>) which has an Al–Si framework with Sr²⁺ guest atoms forms with a narrow composition range of 9.54(6) ≤ <i>x</i> ≤ 10.30(8). Single crystals with composition A₈Al₁₀Si₃₆ (A = Sr, Ba) have been synthesized. Differential scanning calorimetry (DSC) measurements provide evidence for a peritectic reaction and melting point at ∼1268 and ∼1421 K for Sr₈Al₁₀Si₃₆ and Ba₈Al₁₀Si₃₆, respectively. Comparison of the structures reveals a strong correlation between the 24<i>k</i>-24<i>k</i> framework sites distances and the size of the guest cation. Electronic structure calculation and bonding analysis were carried out for the ordered models with the compositions A₈Al₆Si₄₀ (6<i>c</i> site occupied completely by Al) and A₈Al₁₆Si₃₀ (16<i>i</i> site occupied completely with Al). Analysis of the distribution of the electron localizability indicator (ELI) confirms that the Si–Si bonds are covalent, the Al–Si bonds are polar covalent, and the guest and the framework bonds are ionic in nature. The Sr₈Al₆Si₄₀ phase has a very small band gap that is closed upon additional Al, as observed in Sr₈Al₁₆Si₃₀. An explanation for the absence of a semiconducting “Sr₈Al₁₆Si₃₀” phase is suggested in light of these findings

Crystal Chemistry and Physics of UCd₁₁

In the phase diagram U-Cd, only one compound has been identified so farUCd11 (space group Pm3̅m). Since the discovery of this material, the physical properties of UCd11 have attracted a considerable amount of attention. In particular, its complex magnetic phase diagramas a result of tuning with magnetic field or pressureis not well-understood. From a chemical perspective, a range of lattice parameter values have been reported, suggesting a possibility of a considerable homogeneity range, i.e., UCd11–x. In this work, we perform a simultaneous study of crystallographic features coupled with measurements of physical properties. This work sheds light on the delicate relationship between the intrinsic crystal chemistry and magnetic properties of UCd11

BaGe₆ and BaGe_6‑x: Incommensurately Ordered Vacancies as Electron Traps

We report the high-pressure high-temperature synthesis of the germanium-based framework compounds BaGe₆ (<i>P</i> = 15 GPa, <i>T</i> = 1073 K) and BaGe_6–<i>x</i> (<i>P</i> = 10 GPa, <i>T</i> = 1073 K) which are metastable at ambient conditions. In BaGe_6‑<i>x</i>, partial fragmentation of the BaGe₆ network involves incommensurate modulations of both atomic positions and site occupancy. Bonding analysis in direct space reveals that the defect formation in BaGe_6–<i>x</i> is associated with the establishment of free electron pairs around the defects. In accordance with the electron precise composition of BaGe_6‑<i>x</i> for <i>x</i> = 0.5, physical measurements evidence semiconducting electron transport properties which are combined with low thermal conductivity